In recent years, there have been many improvements in automotive drive trains, including improvements relating to the transfer of torque from a prime mover to drive axles. Where four-wheel drive systems are used, transfer cases have been developed which generally provide torque transfer to one output to drive an axle and to another output for driving another axle. Some such transfer cases generally have included a differential mechanism for allowing relative rotation between the outputs. Other such transfer cases have included a freewheel device which automatically engages and disengages the four-wheel drive function, as required.
In two-axle, four-wheel drive vehicles wherein the steering axle wheels are driven through a freewheel device, the known prior art requires that the steering axle wheels overrun the fixed axle wheels until the fixed axle wheels slip enough to equalize wheel speeds, at which time the steering axle wheels assume some of the driving effort. As disclosed herein, it has been found that the steering axle wheels need not overrun and indeed may underrun by a limited amount. One result is that equal axle ratios may be incorporated in the drive train to provide certain advantages in vehicle handling characteristics and operating efficiencies.
It has been found also that under certain operating conditions, existing systems exhibit undesirable characteristics. Specifically, when coasting on slippery surfaces or descending a hill on loose terrain using the engine as a brake, there is a tendency for the fixed axle wheels to skid, with consequent loss of vehicle direction control. As disclosed herein, this tendency may be corrected by providing sufficient drive back from the steering axle wheels to keep the fixed axle wheels rotating.